Chlorine dioxide may be used in a variety of settings, including the following. Chlorine dioxide is used primarily (>95%) for bleaching of wood pulp, but is also used for the bleaching of flour and for the disinfection of municipal drinking water. The Niagara Falls, N.Y. water treatment plant first used chlorine dioxide for drinking water treatment in 1944 for phenol destruction. Chlorine dioxide was introduced as a drinking water disinfectant on a large scale in 1956, when Brussels, Belgium changed from chlorine to chlorine dioxide. Its most common use in water treatment is as a pre-oxidant prior to chlorination of drinking water to destroy natural water impurities that produce trihalomethanes on exposure to free chlorine. Trihalomethanes are suspect carcinogenic disinfection byproducts associated with chlorination of naturally occurring organics in the raw water. Chlorine dioxide is also superior to chlorine when operating above pH 7, in the presence of ammonia and amines and/or for the control of biofilms in water distribution systems. Chlorine dioxide is used in many industrial water treatment applications as a biocide including cooling towers, water processing and food processing. Chlorine dioxide is less corrosive than chlorine and superior for the control of legionella bacteria.
Chlorine dioxide is more effective as a disinfectant than chlorine in most circumstances against water borne pathogenic microbes such as viruses, bacteria and protozoa—including cysts of Giardia and the oocysts of Cryptosporidium. The use of chlorine dioxide in water treatment leads to the formation of the by-product chlorite which is currently limited to maximum of 1 ppm in drinking water in the U.S.A. This EPA standard limits the use of chlorine dioxide in the USA to relatively high quality water or water which is to the treated with iron based coagulants (iron can reduce chlorite to chloride).
Chlorine dioxide can also be used for air disinfection, and was the principal agent used for decontamination of buildings in the United States after the 2001 anthrax attacks. After the disaster of Hurricane Katrina in New Orleans, La. and the surrounding Gulf Coast, chlorine dioxide was used to eradicate dangerous mold from houses inundated by water from massive flooding. Chlorine dioxide is used as an oxidant for phenol destruction in waste water streams, control of zebra and quagga mussels in water intakes and for odor control in the air scrubbers of animal byproduct (rendering plants). Stablilized chlorine dioxide can also be used in an oral rinse to treat oral disease and malodor.
The industrial preparation of chlorine dioxide is energy-intensive and fraught with health and safety issues. Furthermore, due to the instability of ClO2 at high pressures, the gas is often generated where it is to be used. Large-scale production of ClO2 may involve the use of such reagents as concentrated strong acids and/or externally-added oxidants (such as Cl2, H2O2, or hypochlorite). Electrochemical methods can directly oxidize ClO2− to ClO2 by a 1-electron process but require considerable input of electrical energy and may not be applicable in rural or underdeveloped areas of the world. An iron-catalyzed decomposition of ClO2− has been shown to afford ClO2 (in part), but only under very acidic conditions. Since ClO2 is often generated where it is to be used, these hazardous and/or costly methods must be implemented in facilities that are primarily engineered for other purposes.